Group IV semiconductors form an important class of electronic materials which can be used to implement new, high mobility devices. These materials include in particular, Si, Ge and their alloys, (Si.sub.x Ge.sub.1-x) wherein 0&lt;x &lt;1. Most of the research and development on (Si.sub.x Ge.sub.1-x) devices has focused on the creation of high quality heteroepitaxial (Si.sub.x Ge.sub.1-x) layers on Si substrates to take advantage of the higher carrier mobilities provided by Ge (3900.nu.1500 electron mobility cm.sup.2 /V-sec at 300K.degree.) and to engineer bandgap properties. The bandgap of Si is 1.1 eV and Ge is 0.7 eV, therefore, Si.sub.x Ge.sub.1-x alloys can be formed having bandgaps intermediate these values. Stacked or layered structures of Si/Ge alloys can be used to form quantum wells. One of the keypoints of such research is the choice of the alloy composition, which determines both the bandgap and the limits of heteroepitaxy: the larger the amount of Ge, the more difficult the elimination of defects. Another keypoint is the kinetic path and/or the processing sequence by which these materials are grown. Since these heteroepitaxial structures are metastable, the choice of the kinetic path or processing sequence determines the limit on the microstructural perfection of both the interface and the (Si.sub.x Ge.sub.1-x) layer. Once the basic Si.sub.x Ge.sub.1-x structures are grown to a high degree of perfection they still need to be processed further to be made into operational devices. However, the metastability of these structures strongly limits thermal processing required to convert these structures into devices.
Promising results for (Si.sub.x Ge.sub.1-x) manufacturing technology have previously been demonstrated: a (Si.sub.0.8 Ge.sub.0.2)/Si Heterojunction Bipolar Transistor (HBT) with a gain four times larger than that of a Si homojunction bipolar transistor was fabricated at Stanford University in 1988 [C. A. King, J. L. Hoyt, C. M. Gronet, J. F. Gibbons, M. P. Scott and J. Turner, IEEE Elec. Dev. Lett., 10, 52, (1989)]. But little progress has been made in the specific area of processing (Si.sub.x Ge.sub.1-x) heterostructures for the purpose of integrating devices. One important problem is the formation of an adequate passivation layer and a good quality dielectric on heteroepitaxial (Si.sub.x Ge.sub.1-x) structures. Typical passivation layers and dielectrics for silicon based devices are silicon nitrides and silicon oxides. But presently those skilled in the art have been frustrated in attempts to grow oxides and nitrides of Si.sub.x Ge.sub.1-x. For example, if Si.sub.x Ge.sub.1-x structures are subjected to conventional thermal oxidation, the (Si.sub.x Ge.sub.1-x) decompose and the bandgap engineering is lost [O. W. Holland, C. W. White and D. Fathy, Appl. Phys. Lett., 51 (7), 520-2 (1987)]. The authors of the above-reference report that during steam oxidation of Ge ions in Si "the implanted Ge is totally rejected by the growing oxide leading to the formation of an almost pure layer of Ge between the oxide and underlying Si." p. 520. More recently, LeGoues et al. Appl. Phys. Lett. (7) p. 644 (1989) attempted to oxidize Si.sub.x Ge.sub.1-x formed by UHVCVD at 550 C. The samples were oxidized in steam at 800.degree. C. and by dry O.sub.2 at 800.degree. and 1000.degree. C. with the same result--Ge is rejected from the oxide and piled up at the SiO.sub.2 /Si interface.
U.S. Pat. No. 4,800,100 issued 24 January 1989 to Herbots et al. and Herbots et al. in AVS4, AIP Vol. 167, pp 229-250 (1988) "Semiconductor-Based Heterostructure Formation Using Low Energy Ion Beams: Ion Beam Deposition (IBD) and Combined Ion Beam Deposition (CIMD)" disclose a process for growing thin films by using simultaneously, concurrently, or sequentially molecular beam evaporation (MBE) and ion beam deposition (IBD) within a single chamber i.e. without breaking vacuum. They also disclose a number of potential uses of the CIMD process for growing oxides or nitrides, such as, oxides of GaAs; Ge based oxides, and silicon oxides, nitrides, or oxynitrides, formed on germanium or aluminum arsenide substrates with atomically sharp interfaces. In the growth of these materials by CIMD, the oxygen species is provided by the ion beam and the other species by molecular beams.
According to Herbots et al. a typical process would proceed as follows:
(a) GaAs is supplied from a Ga molecular beam and As from molecular beam to form a GaAs film on a substrate.
(b) The molecular beams are switched "off" while, simultaneously, an oxygen or nitrogen ion beam is switched "on" and an Si molecular beam, also switched "on". A low temperature Si oxide or nitride is thereby deposited on the GaAs substrate, forming a heterodielectric structure. Other examples suggested comprise Si oxides or nitrides on Ge, InP, AIGaAs, and CdTe.
Recently, oxidation of stable phase pure Si has been accomplished, "Todorov et al., IEEE Electron Device Letters, Vol. EDL-7, No. 8 Aug. 1986, Herbots et al., AIP Vol. 167 supra. But the formation of oxides and nitrides on a completely metastable material, such as, Si.sub.x Ge.sub.1-x /Si heteroepitaxial films is much more difficult. The Si.sub.x Ge.sub.1-x films decompose under thermal treatment and their behavior is unpredictable due to their non-equilibrium nature. For example, one could have expected that the Si in the Si-Ge alloy would have enhanced thermal oxidation if the SiGe were in the equilibrium phase. However, as noted above, the opposite occurs and the alloy decomposes.
This process limitation is critical if silicon germanium alloys are to be used in Metal Oxide Silicon (MOS) technology for high speed digital electronics. Indeed, if the channel region of a MOS Field Effect Transistor (MOSFET) is made of (Si.sub.x Ge.sub.1-x) to take advantage of the high carrier mobilities of the material, a solution must be found to the problem of growing a reliable dielectric gate material with a good quality interface on top of it.
A need exists therefore for a process of reliably forming an oxide or nitride of a Group IV alloy such as Si.sub.x Ge.sub.1-x for the reasons stated above.